Lecture 6 Flashcards
what external signals do plants respond to?
temperature, wind, CO2, pathogens, water status, mineral nutrients, light levels, light quality, day length, gravity, humidity, herbivores, O2 levels in soil
ability to perceive light and hormones affects what developmental responses?
seed germination, seedling development, shade avoidance, phototropism, stomatal opening, chloroplast movements, flowering,
Role of light:
energy: photosynthesis
information: photomorphogenesis
photoreceptors
detect different wavelengths of light; photoreceptors, signal transduction pathway, response
skotomorphogenesis
light seeking response, etiolated, grow towardslight
etiolation:
hypocotyl becomes long and spindly, seedling is pale yellow (no chloroplast), apical hook forms in dicots,
exposure of skotomorrphogenesis plant to light
etiolated sidelight switches to photomorphogenesis and becomes de-etiolated
de-etiolation
chloroplast development, hypocotyl stops growing, SAM initiates leaf production
SAM
shoot apical meristem
phytochrome receptors:
apoprotein and chromophore
Pr phytochrome
chromophore absorbs red light
Pfr phytochrome
chromophore absorbs far-red light
phytochrome interconversion
in the presence of red or far red light, Pr and Pfr forms interconvert in which the chromophore undergoes a cis-trans isomerization leading to conformational change in the protein part of the phytochrome
R:FR
ratio of red to far red light which is perceived by phytochrome to elicit a response
phytochrome receptor functions
germination–small seeds with small endosperm require light to germinate
seedling establishment–etiolated to de-etiolated or non photosynthetic to photosynthetic or skotomorphogenesis to photomorphogenesis
architecture of plant–shade avoidance
initiation of flowering and seed dormancy
PIFs
polychrome interacting factors; small gene family of transcription factors
competitive shade avoidance response
grow out of shade into sunlight to promote photosynthesis, growth, and reproduction. triggered by low R:FR values because chlorophyll absorbs red light and reflects far-red light
features of shade avoidance response
growth of stem, growth of petioles, inhibition of leaf expansion, early flowering
Trigger of shade avoidance:
in shade a plant will have more Pr than Pfr because of the low R:FR indicating that reduction in active Pfr initiates shade avoidance
what phytochrome are involved in shade avoidance?
PHYB, PHYD, PHYE all act redundantly and have a major role in shade avoidance
PHYB
is a major contributor to shade response in shade and it inhibits shade response in daylight
PHYD and PHYE
contribute less to shade response in shade and they are not involved in inhibiting shade response in daylight
PHYB mutant arabidopsis plant
shade response reduced but still observed in shade
shade responce observed in daylight
response pathway in daylight
R>FR – PHYB Pr absorbs light – interconverts to PHYB P fr – negative regulation occurs – PIFs do not facilitate shade response
response pathway in shade
R
Phototropism
directional growth towards a light source
more cell growth on one side of the stem to bend it
increases opportunity for light capture + photosynthesis
triggered by blue light
PHOT1 and PHOT2 function
gene products of PHOT1 and PHOT2 are required for phototropic response, chloroplast movement and stomatal opening responses. Evidence from PHOT1&2 double mutant which failed to present any of these responses
Phototropic 1&2 receptors description
blue light photoreceptors with redundant functions
plasma membrane associated
kinase domain phosphorylates itself after blue light absorbed
auxin gradient produced by higher auxin on shaded sides promoting cell elongation bending stem towards light
phytohormone ethylene gas functions
seedling germination abscission of leaves and fruits senescence of leaves and fruits response to pathogen attack regulation of fruit ripening
ethylene phytohormone in seedling development
seedling is wounded while pushing through soil, ethylene is produced, ethylene signal transduction pathway is initiated, seedling triple response occurs
seedling triple response
allows seedling to force its way through soil and provides protection for Shoot Apical Meristem SAM
features of seedling triple response
hypocotyl becomes shorter and thicker
roots become shorter and thicker
apical hook becomes exaggerated
genetic mutant screens
screen through many seedling with different but unknown mutations looking for seedlings with a specific mutant such as mutant triple response phenotype
etr1 mutant
no triple response when exposed to ethylene; ethylene-insensitive mutant; encodes a receptor kinase protein that binds to ethylene
ctr1 mutant
triple response in absence of ethylene; constitutive triple response mutant
ethylene stp
(signal transduction pathway) no ethylene–ethylene receptors active–activate CTR1, a negative regulator of ethylene response
CTR1
negative regulator of ethylene STP
describe what happens when ethylene is present
ethylene binds to ethylene receptors, ethylene receptors phosphorylate making them inactive. Inactive receptors are unable to activate CTR1 so CTR1 is in turn unable to inhibit the downstream ethylene STP function resulting in an ethylene response occurring
is ethylene response positively or negatively regulated?
the ethylene receptors negatively regulate CTR1 which negatively regulates the STP. each step is negatively regulated, however the presence of ethylene overall positively regulates the ethylene response.
RAN1
probably supplies ETR1 with copper ions required in ethylene binding site
EIN2
membrane spanning protein downstream of CTR1, function unknown
senescence
aging resulting in deterioration of biological function
what would be observed in a EIN2 loss-of-function mutant in the presence of ethylene for the seedling triple response?
because ein2 is located downstream of CTR1, loss of its function would result in the signal not being transduced causing the seedling to be unable to display the triple response. CHECK THIS ANSWER
temperate climate seeds
undergo maturation drying to promote dormancy and seed longevity. become dormant at end of seed development to prevent fall germination allowing overwinter. embryo survives in seed for long time without water due to seed desiccation and drought tolerance
tropical plant seeds
germinate as soon as seeds mature
ABA
abscisic acid; promotes dormancy and inhibits germination
GA
gibberellin or gibberellin acid; promotes: germination, mobilization of seed reserves, cell expansion in growing seedlings, glowering
VP1
regulates expression of ABA inducible genes during seed maturation in corn and arabidopsis
GA biosynthesis mutant
seeds that don’t germinate
GA insensitive mutants
seeds that germinate poorly
ABA biosynthesis mutants
seeds that germinate sooner than wt
ABA insensitive mutants
seeds that germinate when wt does not
ga mutants
low GA–no germination unless GA is added
aba ga mutants
low GA and low ABA levels–germination occurs without adding GA
ratio of GA to ABA
it is a balance of GA and ABA that regulates germination, not absolute levels; GA and ABA act antagonistically
Outline the process from seed development to seedling development in reference to GA and ABA
- ABA promotes desiccation tolerance and dormancy
- ABA is reduced and GA is increased causing dormancy to end
- GA promotes reserve mobilization
- GA promotes growth and cell expansion during development of seedling
what is actively repressing the triple response genes
Etr1 and CTR1 both actively repress the triple response genes
